Flexographic printing is a method employing flexible reliefs made from rubber or a laminate of rubber and plastic and therefore has such good ink tranferability that it might be possible to print on a rough surface, such as on corrugated cardboards. However, since the reliefs are easily deformed by printing pressure, the thickening of printed images, so-called "dot gain", is liable to occur and thus it is difficult to obtain sharp and good printed images. Accordingly, it is considered that the flexographic printing is inferior to letter press printing or offset printing.
In order to reduce the dot gain, Japanese Kokai Publications 6392/1980 and 47966/1968 propose resin plates for flexographic printing, which provide dots which are difficult to be deformed by printing pressure. Japanese Kokai Publication 970/1990 also proposes a photosensitive resin plate for flexographic printing, which provides smaller relief than a negative image, thus making the relief image smaller than negative film images by the loss occurring when printed. The resin plates of the former patents have a structure comprising two layers which are different in hardness from each other, of which the upper layer to be contacted with paper has a higher hardness than that of the lower layer and the lower layer is softer as absorbing the printing pressure. In this method, however, a platemaking process is complicated because of the two layer construction of the photosensitive layer. In the latter patent's method, an air layer is provided between the negative film and the resin plate and an exposure is then conducted, whereby the oxygen in the air layer inhibits the polymerization reaction in some degree to result in obtaining a relief smaller than the negative image. The method, however, has drawbacks in that it is difficult to handle because the photosensitive resin is liquid and that the resulting dots are fragile.